Primary attention is being devoted to the behavior of grafts that are only incompatible with respect to weak transplantation antigens in mice and rats. A major portion of this effort is being directed toward evaluating the role of Langerhans cells in the rejection of H-Y-incompatible skin isografts in rats and mice by replacing them (via their sojourn on intermediate tolerant hosts) with MHC-compatible and -incompatible female (or male) Langerhans cells. Our preliminary results suggest that MHC restriction is involved in transplantation immunity, and we are seeking further evidence for this. Toward this end, we are also evaluating the survival of MHC-compatible and -incompatible cultured (versus fresh) endocrine tissue allografts in mice and rats.